Production of zirconium nitride



Feb. 8, 1949. P. P. ALEXANDER 2,461,019

PRODUCTION OF ZRCONIUM NITRIDE Filed March 2, 1945 A mf , INVENTR en-w A? MEXAER ATTORNEYS Patented Feb. 8, 1949 L:riminitrillen-or zmooNIUM-'NI'r-RIDE ,kileter iP. ,-Alexander,\-Beverly, Mass., assigner?l l.to MetalsHydrides..-Incorporated, Beverly, Mass., a corporation IofMassa'chusetts ffAppl-ioationMarchZ, 1945;; SerialNoiSOQS-SS .nitrideato apassfaestream of initrogen--orf-ammonia uversaheatedibody `if -thefzirconiumf metalto; be initrided. aIffthe zireoniumimetalsis -in1a,forni of r-lsubstantialf-esize, {there-iis ffa .tendency-:for lthe initridingsiaction to: take:place.A onlyat the surfaee, .If \,thesurface Aof .the czirconium is notfelean sand mclear. of an oxideicoatingffor :exampleltheinitridiingalction-/afis :greatly v`inhibited. To overcome these diculties, the zirconium is-iusedin finely dividedgiorm landzspeeial .lffprecautions lare :taken toessure eleanimetal'lic surfaces ,on-the individual par-tides. -There lis a markedtendency forffusion et :the mon-nitrided zirconium particles-l-to .take placedue-toexcessive exother-mici heat generated by the zirconium that is nitrided. tAsdusionof @the ,nonnitridedzirconium setsX in,- the vparticles fagglomerate and then merge intoaailargeri-body, .which may@ take in .jsubstantially` all .o'f itheipar- -iticlesoof fzireoniumiand: zirconiumnitride. In .-naddition, @the zirconium tendsv to `.fuse .against the .oontainerfinwhichthenitridingireactionis con- '25 ducted. The ynettresult is f to i obtain nan imper- .ffectlyinitridedizirconium .product, 4.contaminated .--Withithametal .nofthe containenin `the V.form of fa '.fusedmass ithatcannotxibe removed ffromwthe `oontainer withoutfgreat diniculty. v-'Ih-ismthod uisiineiiicient..randYcostly.

.eAsiaaresuit vvof myfinvestigationdl have discovreda .relatively'.efeientiandinexpensivernethod offproducing. zireonium.; nitride tof. high.,quality. The necessarylnelyv-,divided .metallicQzireonium `is.ohta1`ned )oy ,theire'duction .ofQainely divided zireoniumrcompoun'diinasuitable reaction chamher. l :Therfr'shlyvreduoed zirconiumparticles are then converted h.to the desired .zirconium...nitride i in* thesameireaction `chamber as1 part ,'o'f the. same operation -ilmiaccordance with theinventiom aicharge J of 'finelyx dividednzirconium oompoundf. such as .the -oxidasehlorde and-iodide, and .a'suitableireducing .=agent;-.suchaas fa;l reducingmetal onreducing metal ,compound fis coniined in :fa :reaction zone. YThe ,charge isheated. to a,:temperaturefsuicientlylhigh to reduce .the1=zirconium compound. Nitrogen or :ammonia .eis .athen-1 admitted y.to `Vthe reactionx zone andthe ;redueed..,zirconium. isconver=.ted @to its nitride. 1 :'Acconding fto; a vpresently;v preferred practice of -the inrention;'.the i-reactionfizone and fchargaare viA-sllbfjnotedrtoia l.modunito reiniovenbectionable aainandirnoismre. .Totaidfin-ithe rernovalfntvl the n fair and moisture theereactionfzoneiandfeharge areradvantageouslyi hea-tedto.- a. suitablewelevated temperaturef;as-'evacu-ationitakes place. While the rednctionnoperationina-3hV be. ocndueted/.Wh-iie Ithe-reaction zenefandaehargezarexnnder veacuurn,

the springing of a leak in the reaction chamber V .lthefchar'nber to the :outside: atmospherezandthus prevent Vairaeandernoisturezfronnseeping finto athe reaction chamber. 'L'Tofmaintainzthis'balancethe positive ipressureiiof '.thezy inertrgasin Eithe; chamber 'is maintained uby :the :admission :roi zfurther L:amounts:off:inert;gas;ifsnecessary.

-' Axnongrthe freducing-ragentszthat :may-doe l' em- 1: ployedf4 to :reduce .atheitfzirconilmr:compounds rare :the A 'aikalinefearthimetals :'t'ca1cium,.:zbarium.1and -fstrontiumandzmaignesium iiirnongathe reduc- --ing 'metalicompounds-:thatz' mayl be. :employedifare 4ithezfredueingrmetalhydririessfsuchf aszthefhydride of the alkaline earth metals, and particularlyrcaly-ciumihydride:fzloec'ause1 Aof fitssravailability. f'Ihe 30 :charge iis;:preferablyzfformed: :ofi intimately rad- -mixedfl and dinely. idirided fiparticles z of :zirconium :compound :and:reducingfzmetalveompounda such flas-xacreducing metal hydride, ;f or examplefcaloiurn hydridexror freducingmetal; the lf-reducing:agent 35.being rnsedlfin ssiightreexcessxin vother -words,

slightly more? than 'thatatheoreticaliyrequired:to

effectmeductiomofzall,oftheezirconium=,compoun presentsin thetcharge. 1 ,Y

The .charge initheareactionizonesis; heatedto; a

s 40 xten'xperature :sutiiciently-:high i vto :initiate and ifcompl'ete thesreductionoperation. 'zSi-nee the-zir-n .r;c'onirnn ecomponn'd and' the?reducingfagenv :are rpresentdn thewoharge inznely.f.dividedsform, the res'ulting reducedvzirconiumisalsdinithe,form/.of

*45 finely divided particles distributed through the :resultingl'mass rofv1ine1yidi-vided compound ofsthe reducingz agent. i

wTo illustrateaspraetice .-Lof .the invention, the fstarting:materialadvantageouslyv.isfnelywdivided `zirconium dioxideipigmentfwhiehfas;made today is .of highlpurity. -sSinceX a reducing :metal such asl metallic calcium, I or example r-foannot aupresent be converted readily into a nely divided form ifor intimate-admiXture with YAthe ylnelyilic'iiizi'ded .tziroonium '.-d-ioxide, itt-iis,v preferable to use 1mag nesium, which can be converted readily into a iinely divided form, for example granules. Furthermore, there is less tendency to generate an excessive amount of heat when the magnesium reacts with the zirconium dioxide, thus eliminating or minimizing the problem of fusing the resulting reaction v mass.

In the case of a charge of intimately adinixed zirconium dioxide pigment and nely divided magnesium metal particles, the reduction operation results in iinely divided zirconium metal particles distributed throughout a mass of refractory magnesium oxide particles. The reaction may be indicated as follows:

Nitrogen or ammonia is admitted to the reaction zone while the charge is still hot to convert the reduced zirconium metal to its nitride. The freshly reduced zirconium particles present substantially clean metallic surfaces, and are therefore in optimum condition for nitriding. The reduced zirconium particles are converted by the nitrogen or ammonia to zirconium nitride. The alternative reactions may be indicated as follows:

There is a considerable evolution of heat as the nitriding reaction takes place so that the temperature of the charge may be quickly elevated. Since the reduced zirconium particles are widely distributed and held more or less separately in the mass of refractory compound of the reducing metal, there is little opportunity for non-nitrided zirconium particles to fuse together. The reduced zirconium particles are widely distributed and held more vor less separately in the resulting mass of refractory magnesium oxide. Even though there is a considerable evolution of heat as the nitriding reaction proceeds and the temperature of the charge rises appreciably, the

fusing together of the zirconium particles is inhibited because they are separated from one another by the protective particles of magnesium oxide.

Due to the presence of a slight excess of the reducing metal, traces of oxygen present in the charge and reaction zone combine with the reducing metal and therefore protect the reduced zirconium as well as the newly formed zirconium nitride. A certain amount of the nitrogen may also combine with any reducing metal left over. For this reason, it isk preferable to use the reducing metal only in slight excess, particularly when it is desired to obtain a high purity zirconium nitride product, although this is not so important in cases where the nitride of the reducing metal may be readily leached from the main zirconium nitride. Thus, magnesium nitride, if formed, may be' leached from zirconium nitride in :the leaching step described below.

In a presently preferred practice, the inert gas is retained in the reaction chamber as the nitrogen or ammonia is admitted. Regulatedamounts of nitrogen or ammonia may then be amitted to regulate the rate of nitriding and hence the rate of generation of exothermic heat. Objectionable overheating of the charge may thus be prevented. Y

To facilitate the reduction operation Yor the nitriding operation, or both, the charge may be mixed as it takes place. A special mixing device may be used for the purpose, or the charge may be suitably tumbled.

If the nitriding agent is ammonia, it dissociates at the high temperatures employed, and the desired nitriding action then takes place. It is thought by some that nascent nitrogen resulting from the vdissociation of the ammonia has a greater ainity for the zirconium metal than normal nitrogen. My investigations lead me to believe that when ammonia is dissociated at high temperatures, it results in pure nitrogen and puie hydrogen; the pure hydrogen reacts with and removes traces of surface oxide on the zirconium metal particles to be nitrided, thus providing clean metallic surfaces; and the pure nitrogen then reacts with the zirconium metal particles to form zirconium nitride. In other words, the hydrogen of the ammonia appears to function as a scavenging agent which purifies the zirconium metal particles for reaction with the nitrogen.

The resulting reaction mass is permitted to cool, say to room temperature, in its atmosphere of nitrogen, after which the zirconium nitride is suitably separated from the compound of thereducing agent. This may be done, for example, by crushing the reaction mass, if necessary, and leaching out the compound of the reducing agent with a suitable solvent, such as a dilute solution of a suitable'acid, for example hydrochloric, sulfuric or acetic acid, or a solution of an ammonium salt. The wet zirconium nitride is then dried and stored for use.

These Vand other features of the invention will be better understood by referring to the accompanying drawing, taken in conjunction with the following description, which illustrates diagrammatically an apparatususable in a practice of the invention.

The apparatus shown comprises a pot retort lil suitably suspended within a furnace l I having a heating chamber l'with a conduit i3 near the bottom through which to introduce'heating gases into the chamber, and a flue opening I4 near the top thereof through which to remove spent gases from the chamber. The retort is advantageously made of heat-resistant steel. It is provided with a removable cover l5, which may be bolted to a flange I6 extending circumferentially around the open end of the retort, a` gasket l-'l being disposed between the two to provide a non-leaking joint. The retort is suspended in the chamber by means of a plurality of spaced supports v|'8 extending circumferentially of the retort between the flange andthe top of the furnace.

The cover is provided with a pipe i9 having a valve I9 for the controlled passage of nitrogen or ammonia, a pipe 20 having a valve 20 connecting the other pipe for the controlled passage of inert gas, such as helium or argon, into the retort; a charging conduit 2| to supply ltitanium compound, such as titanium dioxide pigment, and a reducing agent, such as magnesium, to the interior of the retort. As shown, theV conduit is fitted with three spaced valves 22, 23 and 24,t`o provide an evacuating passageway 25 and a trap 26 of su'lcient size to receive a substantial charge, forY example, of intimately admixed zirconium dioxide pigment and magnesium 21. A pipe 28 with a valve 29 connects the evacuating passageway with a source of vacuum, not shown.

A mechanical mixing device 3U fits Within the interior of the retort. It is provided with a vertical shaft 3l, the upper end of which extends through the cover and is suitably heldin position' Iz-,canora by: a.: hearing 32: andi aa collar.A 33:' securedi the 'Ihemppenendgof theshaitdszaisoienuipped: with a2 pulley-f 34'; connectiblea with ai source? of:

power; ,notshown The: loxvverfpartz ofi theshaitf is.

fitted: with a; plurality` offV laterali supports' 35:@ to Whiciizisr` attached ai helical metallic-f ribbon. '38 adapted.- toi followx the; contourv of' the? imide;` of' the .retort r. so as tonaise charge :materialstconned therein along: the inside surface: of the-.retochandi then:k to ro-llthernv toward thecententhereby ins. timateiygmixingthe:charge materials..

A ldischarge conduit:y ccnnectsf with,5 theibetf tomoffthe'retortandiextendsvthrough theffbottom of? the furnace: A' valve41fis=-provided inthe conduit below: the furnace bottom: forsthe4l removal' ofiresiduesziromthe retort;

Iniacoordancevx Witlr` a practice of: the invention,

assure asealed joint; Heating. gases a-reipassed through conduit I3i'ntoiieating chamber l2, so that the bottom of retort I0 is heated. Spent heating gases: escapee through` flue opening M. Valves: 2,4, 23andl 2.a are opened andvalves I9', 22 and 4l are closed until-,theainmoisture and other gaseous products are evacuated from the interiorrof-Fthe'retorti This may`V be accomplishedgby connecting conduitzZS with a vacuum pump, ,notshown. To hasten-the,A evacuatiomstep, the temperature of the interior2 of thefretortlJ is suitably raised. The charge ma-yf also befmix-ed, by-rotating-,device 30 as it is placedrunder-vacuum tohelpremove air and moisture.,

Gn completion of the evacuation: step, ,valves 24 andz2ii areclosed, valve 22 is'opened and'charge 2:1- of zirconium dioxide andmagnesium;,in slight excesssis passed into conduit 2| and droppedfinto trap 26; after Whichrvalve 22Y isvclosed and valve 29 is openedto'evacuateair and-moisture from tranzZGfandzehar-ge, 21:. Valvesszeiand Ziharezthen closed and valve 24 is opened so that the charge is dropped into the retort.

Additional heating gases are passed into heating chamber i2 to raise the temperature of the charge within retort I!) to a temperature at which the reduction of the zirconium dioxide by the magnesium may proceed satisfactorily. Device 30 is rotated to mix the charge intimately. While the temperature may be checked in various ways, it is practical to employ a hollow shaft 3l, in which is placed a suitable thermoeouple.

In a presently preferred practice, valve 20' is opened, after charge 21 is dropped into retort l0 and valve 24 is closed, to admit inert gas, such as helium or argon, or both, in amount sufficient to place and maintain the reaction zone under substantial positive pressure, for example 5 pounds. The charge is then heated and mixed before and during the reduction reaction.

As the zirconium dioxide particles are reduced, the resulting metallic zirconium particles are distributed throughout the resulting mass of magnesium oxide particles. The magnesium oxide particles tend to separate the reduced zirconium particles. In other Words, the zirconium particles have limited opportunity to contact one another because they are suspended in a mass of magn nesium oxide particles.

Valve I9 is then opened to admit controlled amounts of nitrogen gas or ammonia to the interior of the retort. Unless the temperature of the reaction mass Within the retort is insufficient to initiate the nitriding reaction between the freshly reduced zirconium particles and the nitrogen or ammonia, introduction of further heating gases into the heating chamber is terminated. As

pointediout above; the reaction.betweentzirooniumand nitrogen is an exothermic one and, unless; speciali precautions-aref taken, objeetionablefoverheating tends; to: take? piace. thereby? causing,- fusionyofithenzirconium: particlesy andimpairment of? the nitriding. reactionz. o Toi prevenir tlriisfr un, desirable: result, the zirconium: particles;A are. nitrideel;as5 they-f ares'distributed'sin: and e amongV the magnesium'. oxidet particles; In appreferred'ipracf ticegtiie inert; gasis retained in; the retort sonthat7 the-nitrogeni or. ammonia-,may be admixedjand-v diluted therewith; Iintheinert gas: ini the retort: is maintainedataneoptimum pressure; the amountt ofrexothermic heat; released by` the nitriding ,ree action` maybe icontrollediby. regulatingithe rate sat. which nitrogen; orf' ammonia: is: admitted' to'fthe. ret'orti.` 'llliisV :may: bef done: automatically,m by.vv feedingiadditional amounts; of: nitrogenecnammoniarto the` retortV when the-f pressure withinzthe retort 'falls below: a predetermined lei/el,v but lnoir; ini

amount; toi exceed ae predeterminedz level;Y the gen1. or: ammoniaA` to. an: amount which". im insuffcient'f-toraise'l the temperature toarn objection:y

v able@ extent: suicient` amount. ofznitrogemoi Y ammonia; isathusadmitted'zto effect; complete version, off; the' reduced zirconium: to: zirconium nitride.Y Thenitridingzreactionzisxcompletechwhen there:` is nrV call; for: additional; nitrogen: or: monia. 1 A

The-retortamdits;contents are thenxpermitted to cool: Valvesg IS' andZilff are closedandcover Pi isa-removed. The-vzirconium, nitride andimagf' nesium; oxide are then removed from;v the retort; Thissm ayfbef.-doneaby openingvalve; 4 L andvrotatf ingfthe; mixing device; when; the, reactionLmassis; discharge-d through, conduit: 401. "imei-reaction mass isl crushed,-` if,` desired.,l and leached with dilute acid; such as hydrochloric,. sulfuric on acetic acid, or a solution of an ammonium salt. The acid or ammonium salt dissolves out the magnesium oxide, as well as any magnesium nitride that may have formed, leaving the zirconium nitride, which 'is dried and stored for use.

It will be clear to those skilled in this art that the above example is only by way of illustration; that the practice of the invention readily lends itself to a number of useful modications; and that the invention offers a relatively eilicient and inexpensive method of producing zirconium nitride. y Y

Reference may be'V made to my three (3) c0- pending applications filed March 2, 1945, which also relate to metal nitrides: Serial Nos. 580,687; 580,688; and 580,690; the claims of which are directed broadly to the production of metal nitrides and specically to the nitrides of titanium and tantalum, respectively.

I claim:

l. A method for producing zirconium nitride which comprises confining in a reaction zone a .Y charge of an intimate mixture in nely divided form of zirconium oxide and a metal containing reducing agent capable of forming a refractory a gas selected from the group consisting of nitrogen and ammonia, and converting the zirconium metal to zirconium nitride while disf 7. tributed land held in the protective refractory oxide.

2. A method lfor producing zirconium nitride which comprises confining in a reaction zone a charge of an intimate mixture in iiinely divided form of zirconium oxide and a metal hydride, heating the charge suiiiciently high to reduce the zirconium oxide by reaction with said metal hydride and form a reaction mass in which iinely divided particles of zirconium metal are distributed and held in a mass of particles of refractory oxide, then admitting to the reaction zone a gas selected from the group consisting of nitrogen and' ammonia, and converting the zirconium metal to zirconium nitride while distributed and held in the protective refractory oxide.

3. A method for producing zirconium nitride which comprises confining in a reaction zone a charge of an intimate mixture in iinely divided form of zirconium oxide and a reducing metal, heating the charge sufficiently high to reduce the zirconium oxide by reaction with said reducing metal and form a reaction mass in which iinely dvidedparticles of zirconium metal are distrib# uted and held in a mass of particles of refractory oxide, then admitting to the reaction zone a gas selected from the group consistingof nitrogen and ammonia, and converting the zirconium metal to zirconium nitride while distributed and held in the protective refractory oxide.

4, A method for producing zirconium nitride which comprises conning in a reaction zone a charge of an intimate mixture in nely divided form of zirconium oxide `and calcium hydride, heating the charge suiiiciently high to reduce the zirconium oxide by reaction with said calcium hydride and form a reaction mass in which nely divided particles of zirconium metal are distributed and held in a mass of particles of refractory oxide, then admitting to the reaction zone a gas selected from the group consisting of nitrogen and ammonia, and converting the zirconium metal to zirconium nitride While distributed and held in the protective refractory oxide.

5. A method for producing zirconium nitride which comprises conning in a reaction zone a charge of an intimate mixture in nely divided form of zirconium oxide and magnesium, heating the charge suiiciently high to reduce the zirconium oxide by reaction With said magnesium and form a reaction mass in which iinely divided particles of zirconium metal are distributed and held in a mass of particles of refractory oxide, then admitting to the reaction zone a gas selected from the group consisting of nitrogen and ammonia, and converting the zirconium metal 'to zirconium nitride While distributed and held in the protective refractory oxide.

PETER P. ALEXANDER.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 996,032 Serpek June 20, 1911 1,077,712 Heyder Nov. 4, 1913 1,088,909 Kuzel Mar. 3, 1914 1,180,840 Giulini Apr. 25, 1916 1,274,797 Shoeld Aug. 6, 1918 1,343,441 Farup June 15, 1920 1,366,720 DeLoisy Jan. 25, 1921 1,533,505 Lubowski Apr; 14, 1925 1,803,720 Miner May 5, 1931 v OTHER REFERENCES Chemical Abstracts, voi. 35, 1941, page 47121. 

